Process for separating hydrocarbon compounds

ABSTRACT

Disclosed herein are processes for producing and separating ethane and ethylene. In some embodiments, an oxidative coupling of methane (OCM) product gas comprising ethane and ethylene is introduced to a separation unit comprising two separators. Within the separation unit, the OCM product gas is separated to provide a C 2 -rich effluent, a methane-rich effluent, and a nitrogen-rich effluent. Advantageously, in some embodiments the separation is achieved with little or no external refrigeration requirement.

CROSS REFERENCE

This application is a continuation application of U.S. patentapplication Ser. No. 13/739,954, filed Jan. 11, 2013, which applicationclaims the benefit of U.S. Provisional Patent Application Ser. No.61/586,711, filed Jan. 13, 2012, each of which is incorporated herein byreference in its entirety.

BACKGROUND

Technical Field

This disclosure generally relates to selectively separating carboncompounds containing at least two carbon atoms from a mixed gas streamprovided by a chemical process.

Description of the Related Art

The modern petrochemical industry makes extensive use of cracking andfractionation technology to produce and separate various desirablecompounds from crude oil. Cracking and fractionation operations areenergy intensive and generate considerable quantities of greenhousegases. The gradual depletion of worldwide petroleum reserves and thecommensurate increase in petroleum prices places extraordinary pressureon refiners to minimize losses and improve efficiency when producingproducts from existing feedstocks, and also to seek viable alternativefeedstocks capable of providing affordable hydrocarbon intermediates andliquid fuels to downstream consumers.

Methane provides an attractive alternative feedstock for the productionof hydrocarbon intermediates and liquid fuels due to its widespreadavailability and relatively low cost when compared to crude oil.Worldwide methane reserves are estimated in the hundreds of years atcurrent consumption rates and new production stimulation technologiespromise to make formerly unattractive methane deposits commerciallyviable.

Used in the production of polyethylene plastics, polyvinyl chloride,ethylene oxide, ethylene chloride, ethylbenzene, alpha-olefins, linearalcohols, vinyl acetate, and fuel blendstocks such as but not limited toaromatics, alkanes, alkenes, ethylene is one of the most importantcommodity chemical intermediates currently produced. With economicgrowth in developed and developing portions of the world, demand forethylene and ethylene based derivatives continues to increase.Currently, ethylene production is limited to high volume production as acommodity chemical in a relatively large steam cracker or otherpetrochemical complex setting due to the high cost of the crude oilfeedstock and the large number of hydrocarbon byproducts generated inthe crude oil cracking process. Producing ethylene from far moreabundant and significantly less expensive natural gas provides anattractive alternative to ethylene derived from crude oil.Oligomerization processes can be used to further convert ethylene intolonger chain hydrocarbons such as C₆ and C₈ hydrocarbons useful forpolymer gasoline and high value specialty chemicals.

The conversion of methane to longer chain hydrocarbons, particularlyalkenes such as ethylene, produces a product gas containing multiplebyproducts, unreacted feedstock gases, and inert components in additionto ethylene. The ability to selectively and economically produce andseparate methane based alkenes on a commercially viable scale provides apathway to a significant new source of ethylene useful for production ofethylene based derivatives.

BRIEF SUMMARY

As noted above, the present disclosure is directed to methods forproviding C₂ carbon compounds via oxidative coupling of methane (OCM).The methods may be summarized as including steps of:

a) combining a feedstock gas comprising methane with an oxygencontaining gas comprising oxygen;

(b) contacting the combined feedstock gas and oxygen containing gas witha catalyst and providing an OCM product gas comprising ethane andethylene (C₂);

(c) compressing the OCM product gas;

(d) condensing at least a portion of the OCM product gas to provide anOCM product gas condensate comprising mostly water;

(e) introducing the OCM product gas condensate to a first separator;

(f) isentropically expanding and reducing the temperature of a firstportion of the OCM product gas;

(g) introducing the first portion of the OCM product gas to the firstseparator and introducing a second portion of the OCM product gas to asecond separator, the second separator operating at a lower pressure andtemperature than the first separator;

(h) removing a C₂-rich effluent and a methane/nitrogen containing gasmixture from the first separator;

(i) introducing the methane/nitrogen containing gas mixture to thesecond separator; and

(j) removing a methane-rich effluent and a nitrogen-rich effluent fromthe second separator.

In certain embodiments of the disclosed methods, the oxygen containinggas is compressed air having an oxygen content of about 21 mol % and anitrogen content of about 78 mol %; and the nitrogen content in themethane/nitrogen containing gas removed from the first separator may beat least about 85 mol %. In yet other embodiments, the first and secondseparators may operate at a below ambient temperature; and adiabaticexpansion of at least one of the OCM product gas, a methane gas, anitrogen gas, or a methane/nitrogen gas mixture may provide at least aportion of the cooling to produce the below ambient temperature. Inother embodiments, the oxygen containing gas is compressed oxygen havingan oxygen content of at least about 90 mol % and a nitrogen content ofat most about 10 mol %; and the nitrogen content in the methane/nitrogencontaining gas removed from the first separator may be at most about 85mol %. In yet other embodiments, the first and second separators mayoperate at a below ambient temperature; and the compressed oxygen may besupplied via a cryogenic process and the cryogenic process may provideat least a portion of the cooling to produce the below ambienttemperature.

Methods for providing C₂ carbon compounds via oxidative coupling ofmethane (OCM) in accordance with embodiments described herein mayfurther include introducing at least a portion of the methane-richeffluent removed from the second separator to the feedstock gas prior tocombining the feedstock gas with the oxygen containing gas. In certainembodiments, the C₂-rich effluent may include at least about 90.0 mol %C₂, the methane-rich effluent may include at least about 92.0 mol %methane, and the nitrogen-rich effluent may include at least about 85.0mol % nitrogen.

In other embodiments disclosed herein, methods for providing C₂ carboncompounds via oxidative coupling of methane (OCM) may further includereducing water content in the OCM product gas to about 0.001 mol % atmost prior to condensing at least a portion of the OCM product gas.

Methods for providing C₂carbon compounds via oxidative coupling ofmethane (OCM) in accordance with disclosed embodiments may furtherinclude reducing carbon dioxide content in the OCM product gas to about5 ppm at most prior to condensing at least a portion of the OCM productgas.

Other embodiments of methods for providing C₂ carbon compounds viaoxidative coupling of methane (OCM) may further include reducinghydrogen sulfide content in the feedstock gas to about 5 ppm. In certainembodiments disclosed herein, the feedstock gas may include at leastabout 20 mol % methane and compressing the OCM product gas may includeincreasing the pressure of the OCM product gas to at least about 100pounds per square inch gauge (psig).

In another aspect of the disclosed subject matter processes forseparating C₂ compounds from a product of an oxidative coupling ofmethane (OCM) process may be summarized as including:

(a) providing an OCM product gas from an OCM process, the OCM productgas comprising ethane and ethylene;

(b) compressing the OCM product gas to a pressure of at least about 200pounds per square inch gauge (psig);

(c) reducing the temperature of the OCM product gas and condensing atleast a portion of the OCM product gas to provide an OCM product gascondensate;

(d) separating the OCM product gas condensate from the OCM product gas;

(e) introducing the OCM product gas condensate to a first separator;

(f) separating the OCM product gas separated from the OCM product gascondensate into a first portion and a second portion and isentropicallyexpanding the first portion of the OCM product gas through aturboexpander to reduce the temperature of the first portion of the OCMproduct gas;

(g) introducing the first portion of the OCM product gas to the firstseparator;

(h) removing a C₂-rich effluent from the first separator;

(i) removing a first separator overhead gas from the first separator;

(j) reducing the temperature of the first separator overhead gas;

(k) introducing the cooled first separator overhead gas to a secondseparator;

(l) removing a methane-rich effluent from the second separator; and

(m) removing a nitrogen-rich effluent from the second separator.

In additional embodiments of the present disclosure, methods forseparating C₂ compounds from a product of an oxidative coupling ofmethane (OCM) process may further include:

(n) reducing the temperature of the second portion of the OCM productgas;

(o) adiabatically expanding the second portion of the OCM product gas toprovide an at least partially condensed mixed stream that includes asecond OCM product gas condensate and a second OCM product gas;

(p) introducing the at least partially flashed second OCM product gascondensate to the first separator; and

(q) reducing the temperature of the second OCM product gas andintroducing the second OCM product gas to the second separator.

Methods for separating C₂ compounds from a product of an oxidativecoupling of methane (OCM) process in accordance with disclosedembodiments of this aspect of the present disclosure may further includereducing water concentration in the OCM product gas to about 0.001 molepercent (mol %) at most and more preferably to about 0.0001 mol % (1ppmv) at most prior to condensing at least a portion of the OCM productgas.

In other embodiments, methods for separating C₂ compounds from a productof an oxidative coupling of methane (OCM) process may further includereducing carbon dioxide concentration in the OCM product gas to about 10ppmv at most prior to condensing at least a portion of the OCM productgas.

In other embodiments, methods for separating C₂ compounds from a productof an oxidative coupling of methane (OCM) process may further includereducing acetylene concentration in the OCM product gas to about 1 partper million by volume (ppmv) at most prior to condensing at least aportion of the OCM product gas or reducing the acetylene concentrationin a C₂-rich effluent provided by the separations unit to about 1 ppmvat most.

In accordance with other disclosed embodiments of this aspect of thepresent disclosure, methods for separating C₂ compounds from a productof an oxidative coupling of methane (OCM) process may further includereducing hydrogen sulfide concentration in the OCM process to about 5ppm at most using sulfur removal process, system and/or device, such asa sulfur trap. In other embodiments, providing the OCM product gas mayinclude combining compressed air comprising oxygen and nitrogen andhaving an oxygen concentration of at least about 21 mol % with afeedstock gas comprising methane and having a methane concentration ofat least 50 mol % and introducing the combined compressed air andfeedstock gas to at least one OCM reactor. In other embodiments, the OCMproduct gas may include about 90 mol % or less nitrogen and providingthe OCM product gas may include combining an oxygen containing gascomprising compressed oxygen and having an oxygen concentration of atleast about 90 mol % with a feedstock gas comprising methane and havinga methane concentration of at least about 50 mol % and introducing thecombined compressed oxygen and feedstock gas to at least one OCMreactor. In certain embodiments, the OCM product gas may include about10 mol % or less nitrogen.

In yet other embodiments, the method for separating C₂ compounds from aproduct of an oxidative coupling of methane (OCM) process disclosedherein may further include recycling at least a portion of themethane-rich effluent from the second separator to an OCM reactor. Inother embodiments, the C₂-rich effluent may include at least about 90mol % C₂, the methane-rich effluent may include at least about 60 mol %methane, and the nitrogen-rich effluent may include at least about 50mol % nitrogen.

In yet another aspect of the disclosed subject matter, processes forseparating C₂ compounds from a product of an oxidative coupling ofmethane (OCM) process may be summarized as including:

(a) combining a feedstock gas comprising methane with an oxygencontaining gas;

(b) contacting the combined feedstock gas and oxygen containing gas witha catalyst and providing an OCM product gas having a C₂ concentration offrom about 0.5 mol % to about 20 mol %, a methane content of about 60mole percent (mol %) or less, and a nitrogen content of at least about20 mol %;

(c) compressing the OCM product gas; and separating the OCM product gasinto a C₂-rich effluent; a methane-rich effluent; and an nitrogen-richeffluent.

In accordance with embodiments of this aspect of the disclosed subjectmatter, separating the OCM product gas into the C₂-rich effluent; themethane-rich effluent; and the nitrogen-rich effluent may occur at alower than ambient temperature. In addition, in accordance withdisclosed embodiments, adiabatic expansion of at least one of the OCMproduct gas, a methane gas, a nitrogen gas, or a methane/nitrogen gasmixture may provide at least a portion of the cooling to achieve thelower than ambient temperature and in other embodiments of the disclosedsubject matter, adiabatic expansion of at least one of the OCM productgas, a methane gas, a nitrogen gas, or a methane/nitrogen gas mixturemay provide all of the cooling to achieve the lower than ambienttemperature.

In other embodiments, processes for separating C₂ compounds from aproduct of an oxidative coupling of methane (OCM) process may furtherinclude recycling at least a portion of the methane-rich effluent andcombining it with the feedstock gas and/or the oxygen containing gas. Inother embodiments, the C₂+ rich effluent may include at least about 90mol % C₂+ compounds, the methane-rich effluent may include at leastabout 60 mol % methane, and the nitrogen-rich effluent may include atleast about 50 mol % nitrogen. In other embodiments, compressing the OCMproduct gas may include increasing the pressure of the OCM product gasto at least about 200 pounds per square inch gauge (psig).

In another aspect of the disclosed subject matter, processes forseparating ethylene from a product of an oxidative coupling of methane(OCM) process may be summarized as including:

(a) reducing the hydrogen sulfide content of a feedstock gas comprisingmethane to about 5 ppm at most;

(b) combining the feedstock gas with an oxygen containing gas comprisingoxygen;

(c) passing the combined feedstock gas and oxygen containing gas acrossa catalyst to provide an OCM product gas having an ethylene content ofabout 0.5 mol % or greater, a hydrogen content of from about 0.0 mol %to about 4.0 mol %, a methane content of about 95 mol % or less, and anitrogen content of at least about 1 mol %;

(d) compressing the OCM product gas; and

(e) separating the OCM product gas into a ethylene-rich effluent; amethane-rich effluent; and an nitrogen-rich effluent.

In accordance with embodiments of this aspect of the disclosed subjectmatter, the OCM product gas exiting the OCM reactor may be at atemperature of no more than about 1750° F. (950° C.) or preferably nomore than about 1650° F. (900° C.) and/or at a pressure of no more than200 psig (690 kPa). In accordance with other embodiments, the catalystmay include a compound including at least one of an alkali metal, analkaline earth metal, a transition metal, and a rare-earth metal.

In accordance with other embodiments, separating the OCM product gasinto an ethylene-rich effluent; a methane-rich effluent; and anitrogen-rich effluent may include:

(f) reducing the temperature of the OCM product gas and condensing atleast a portion of the OCM product gas to provide an OCM product gascondensate;

(g) separating the OCM product gas condensate from the OCM product gas;

(h) introducing the OCM product gas condensate to a first separator;

(i) separating the OCM product gas separated from the OCM product gascondensate into a first portion and a second portion and isentropicallyexpanding the first portion of the OCM product gas through aturboexpander to reduce the temperature of the first portion of the OCMproduct gas;

(j) introducing the first portion of the OCM product gas to the firstseparator;

(k) removing the ethylene-rich effluent from the first separator;

(l) removing a first separator overhead gas from the first separator;

(m) reducing the temperature of the first separator overhead gas;

(n) introducing the cooled first separator overhead gas to a secondseparator;

(o) removing the methane-rich effluent from the second separator; and

(p) removing the nitrogen-rich effluent from the second separator.

In accordance with other embodiments of this aspect of the presentdisclosure, processes for separating ethylene from a product of anoxidative coupling of methane (OCM) process may further includerecycling at least a portion of the methane-rich effluent from thesecond separator to the reduced hydrogen sulfide content feedstock gas.In other embodiments, the nitrogen-rich effluent may include about 50mole percent (mol %) or greater nitrogen concentration, the methane-richeffluent may include about 60 mol % or greater methane concentration,and the ethylene-rich effluent may include from about 10 mol % to about60 mol % or greater ethylene concentration. In additional embodiments,separating the OCM product gas into the ethylene-rich effluent; themethane-rich effluent; and the nitrogen-rich effluent may occur at alower than ambient temperature and adiabatic expansion of at least oneof the OCM product gas, a methane gas, a nitrogen gas, or amethane/nitrogen gas mixture may provide at least a portion of thecooling to provide the lower than ambient temperature.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

In the drawings, identical reference numbers identify similar elementsor acts. The sizes and relative positions of elements in the drawingsare not necessarily drawn to scale. For example, the shapes of variouselements and angles are not drawn to scale, and some of these elementsare arbitrarily enlarged and positioned to improve drawing legibility.Further, the particular shapes of the elements as drawn, are notintended to convey any information regarding the actual shape of theparticular elements, and have been solely selected for ease ofrecognition in the drawings.

FIG. 1 is a block flow diagram depicting a methane based C₂ productionand separation process, according to one illustrated embodiment;

FIG. 2 is a block flow diagram depicting a methane based C₂ production,treatment, and separation process, according to one illustratedembodiment;

FIG. 3 is a basic process flow diagram depicting a methane based C₂production and separation process, according to one illustratedembodiment;

FIG. 4 is a basic process flow diagram depicting a methane based C₂production, treatment and separation process, according to oneillustrated embodiment;

FIG. 5 is a process flow diagram depicting a separation process usefulfor separating a mixed product gas stream resulting from a methane basedC₂ production process, according to one illustrated embodiment;

FIG. 6 is a process flow diagram depicting another separation processuseful for separating a mixed product gas stream resulting from amethane based C₂ production process, according to one illustratedembodiment;

FIG. 7 is a process flow diagram depicting a detail heat exchangingscheme of another separation process useful for separating a mixedproduct gas stream resulting from a methane based C₂ production process,according to one illustrated embodiment; and

FIG. 8 is a block flow diagram depicting another methane based C₂production and separation process, according to one illustratedembodiment.

DETAILED DESCRIPTION

In the following description, certain specific details are set forth inorder to provide a thorough understanding of various disclosedembodiments. However, one skilled in the relevant art will recognizethat embodiments may be practiced without one or more of these specificdetails, or with other methods, components, materials, etc. In otherinstances, well-known structures associated with specific unitoperations, such as fluid transport, heat transfer, mass transfer,thermodynamic processes, and mechanical processes, e.g., fluidtransportation, filtration, evaporation, condensation, gas absorption,distillation, extraction, adsorption, drying, gas liquefaction, andrefrigeration have not been shown or described in detail to avoidunnecessarily obscuring descriptions of the embodiments.

Unless the context requires otherwise, throughout the specification andclaims which follow, the word “comprise” and variations thereof, suchas, “comprises” and “comprising” are to be construed in an open,inclusive sense, that is as “including, but not limited to.”

Reference throughout this specification to “one embodiment” or “anembodiment” means that a particular feature, structure or characteristicdescribed in connection with the embodiment is included in at least oneembodiment. Thus, the appearances of the phrases “in one embodiment” or“in an embodiment” in various places throughout this specification arenot necessarily all referring to the same embodiment. Furthermore, theparticular features, structures, or characteristics may be combined inany suitable manner in one or more embodiments.

As used in this specification and the appended claims, the singularforms “a,” “an,” and “the” include plural referents unless the contentclearly dictates otherwise. It should also be noted that the term “or”is generally employed in its sense including “and/or” unless the contentclearly dictates otherwise.

As used in the specification and the appended claims, references aremade to a “feedstock gas.” It is understood that a feedstock gas mayinclude any gas or gasified liquid containing methane and recognizableby one of ordinary skill in the art as being suitable for providingmethane to a oxidative coupling of methane (OCM) reaction. As used inthe specification and the appended claims, references are made to an“effluent.” It is understood that an effluent may include any materialor compound either removed or intended for removal from a particularlocation. Additionally, references are made to compositions that arevariously described as being “nitrogen-rich,” “methane-rich,” and“C₂-rich.” It should also be understood that the use of the suffix“-rich” indicates the compound or compounds having the greatest molarconcentration within the composition. For example, a “nitrogen-richeffluent” describes an effluent where nitrogen has the greatest molarconcentration. Similarly a “methane-rich gas” describes a gas wheremethane has the greatest molar concentration. As used in thespecification and the appended claims, references are made to a “unit.”It is understood that a unit may include any number of individual orcombined unit operations such as separation, heating, cooling,condensation, vaporization, and the like as recognizable by one ofordinary skill in the art as being suitable or beneficial for achievingthe indicated results. For example a “separation unit” may have morethan one physical separator and may also include multiple ancillaryheating, cooling, condensation and vaporization unit operations toachieve the desired separation.

As used herein the terms “C₂” and “C₂ compounds” refer to alkane (i.e.,ethane) and alkene (i.e., ethylene) hydrocarbons and not to alkyne(i.e., acetylene) hydrocarbons comprising 2 carbon atoms in theirbackbone. C2+ refer to 2 chain length hydrocarbons and higherhydrocarbon chain length comprising both alkanes and alkenes, e.g.propane and propylene. As used herein the term “C₂ content” refers tothe concentration of C₂ compounds (i.e., ethane+ethylene) present at thespecified location.

The headings and Abstract of the Disclosure provided herein are forconvenience only and do not interpret the scope or meaning of theappended claims or disclosed embodiments.

FIG. 1 is a block flow diagram depicting an illustrative C₂ productionand separation process 100 including one or more oxidative coupling ofmethane (OCM) reactors 102 and one or more separation units 104. Inprocess 100, the pressure of an oxygen containing gas 106 is increased,for example using one or more compressors 108, and the resultant higherpressure oxygen containing gas is combined with a feedstock gas 112containing methane to provide a feedstock gas/oxygen containing gasmixture 110. The feedstock gas/oxygen containing gas mixture 110 isintroduced to the one or more OCM reactors 102. Within the one or moreOCM reactors 102, methane present in the feedstock gas and the oxygenpresent in the oxygen containing gas are passed over a catalystpromoting the formation of an OCM product gas 114 including ethylene andethane. The OCM product gas 114 may also contain amounts of unreactedfeedstock such as methane; inert compounds such as nitrogen; andbyproducts such as hydrogen, water vapor, and various carbon oxides(CO_(x)).

In the embodiment of FIG. 1, the pressure of the OCM product gas 114 isincreased, for example using one or more compressors 116 prior tointroduction to the one or more separation units 104. Within the one ormore separation units 104, at least three effluents are produced: amethane-rich effluent 120, a nitrogen rich effluent 122, and a C₂-richeffluent 124. At least a portion of the methane-rich effluent 120 may berecycled to the feedstock gas 112 or alternatively the methane-richeffluent 120 may not be recycled to the feedstock gas 112. Recycling ofmethane-rich effluent 120 is beneficial since methane is the feedstockfor the production of C₂s, however recycling at least a portion of themethane-rich effluent 120 provides additional operational and economicbenefits because the methane-rich effluent 120 can be utilized withouthaving to meet the stringent requirements of a fungible product, forexample a maximum nitrogen limit imposed on methane intended forinjection into a natural gas transport or distribution systems.

The C₂-rich effluent 124 contains the desired ethane and ethylenecompounds as well as C₃ and heavier hydrocarbon compounds such aspropane and propylene (i.e., C₃₊ compounds). In some instances the C₂compounds, particularly the ethylene, present in the C₂-rich effluent124 can be separated, for example using a C₂ splitter to selectivelyseparate ethylene from ethane, and marketed as a commodity chemical. Inother instances, all or a portion of the ethylene can be introduced toone or more additional unit operations, for example an oligomerizationprocess to create oligomers, such as C₆ (trimer) and C₈ (tetramer)compounds, useful for example in liquid fuel products.

The oxygen containing gas 106 can include any source of oxygen such asair, purified oxygen, or mixtures thereof. The oxygen containing gas 106can be an enriched oxygen containing gas sourced partially or whollyfrom an air separation plant or an air separation unit. The pressure ofthe oxygen containing gas 106 may be increased, for example using one ormore compressors 108, to provide the higher pressure oxygen containinggas. In some embodiments, the temperature of the higher pressure oxygencontaining gas can be adjusted, for example through the use of anintercooler and/or aftercooler installed and operated in conjunctionwith the one or more compressors 108. The addition of stoichiometricquantities of oxygen to the one or more OCM reactors via the oxygencontaining gas 106 can limit the formation of undesirable combustionbyproducts such as CO_(x) within the one or more OCM reactors 102. Insome instances, the temperature of the oxygen containing gas 106 may beincreased, for example by thermally contacting the oxygen containing gas106 with one or more higher temperature gases or liquids, prior tomixing with the feedstock gas 112.

The composition of the oxygen containing gas 106 can vary dependent uponthe source of the gas. For example, where air is used to provide theoxygen containing gas, an oxygen content of about 21 mol % and anitrogen content of about 78 mol % is provided. In at least someimplementations, one or more inert gases, such as nitrogen, argon, orhelium may be present in trace or larger quantities in the oxygencontaining gas 106. Where purified oxygen is used to provide the oxygencontaining gas, an oxygen content of greater than about 21 mol % ispossible. The oxygen content of the oxygen containing gas 106 can beabout 21 mol % or greater; about 40 mol % or greater; about 60 mol % orgreater; or about 80 mol % or greater. Similarly, the nitrogen contentof the oxygen containing gas 106 will vary dependent upon the sourceproviding the oxygen containing gas 106. The nitrogen content of theoxygen containing gas can be about 78 mol % or less; about 60 mol % orless; about 40 mol % or less; or about 20 mol % or less. In at leastsome implementations, the nitrogen content of the oxygen containing gascan be from about 5 mole percent (mol %) to about 95 mol %; about 10 mol% to about 90 mol %; about 15 mol % to about 85 mol %; about 20 mol % toabout 80 mol %; or about 25 mol % to about 75 mol %. The pressure of thecompressed oxygen containing gas 110 can vary. For example, the pressureof the compressed oxygen containing gas can be about 300 psig (2100 kPa)or less; about 200 psig (1400 kPa) or less; or more preferably about 100psig (700 kPa) or less.

The feedstock gas 112 includes methane, all or a portion of which mayinclude methane from relatively clean sources such as that availablefrom a pipeline, commercial or industrial supply or distributionnetwork. In some instances, all or a portion of the feedstock gas 112may be sourced from so called “dirty” sources such as extracted naturalgas that contains contaminants or impurities requiring removal prior tointroducing the feedstock gas 112 to the one or more OCM reactors 102.While in general, the use of a feedstock gas 112 having a known, fixedmethane composition is preferred, gases having a variable methanecomposition may also be used to provide all or a portion of thefeedstock gas 112. Similarly, while the use of a feedstock gas 112having a high methane content is preferred, gases having low methanecontent may also be used to provide all or a portion of the feedstockgas 112 provided any components detrimental to catalyst life, catalystperformance, or any components promoting undesirable side reactions orthe formation of undesirable products are partially or completelyremoved prior to introducing the feedstock gas 112 to the one or moreOCM reactors 102. The methane content of the feedstock gas 112 can varyand be about 20 mol % or less, about 35 mol % or less, about 50 mol % orless, about 80 mol % or less; about 90 mol % or less; about 95 mol % orless; or about 99 mol % or less.

Contaminants present in the feedstock gas 112 can include heavier weighthydrocarbons, acid gases such as carbon dioxide and hydrogen sulfide,nitrogen, water vapor, natural gas condensate (“casinghead gasoline”),and mercury to name a few. The feedstock gas 112 can be pretreated usingknown techniques prior to introduction to the one or more OCM reactors102 to remove some or all of the contaminants such as hydrogen sulfideand heavier weight hydrocarbons that are capable of promoting theformation of undesired reaction side- or by-products, and/ordetrimentally affecting the performance of the OCM catalyst disposedwithin the one or more OCM reactors 102. After treatment, the hydrogensulfide content of the feedstock gas 112 can be about 20 ppm or less;about 10 ppm or less; about 5 ppm or less; or about 1 ppm or less. Aftertreatment the heavier weight hydrocarbons content of the feedstock gas112 can be about 0.1 mol % or less; about 0.05 mol % or less; or about0.01 mol % or less. Since the one or more OCM reactors 102 operate at anelevated temperature, the temperature of the feedstock gas 112 may beincreased prior to mixing with the oxygen containing gas 106 to lessenthe thermal input required to raise the temperature of the feedstockgas/oxygen containing gas mixture to the desired reaction temperaturewithin the one or more OCM reactors 102.

In at least some embodiments the temperature and/or pressure of thefeedstock gas 112 can be adjusted prior to mixing with the oxygencontaining gas 106 or introduction to the one or more OCM reactors 102.The pressure and temperature of the feedstock gas 106 can vary. Forexample the pressure of the feedstock gas 112 can be about 150 psig(1035 kPa) or less; about 100 psig (690 kPa) or less; about 75 psig (520kPa) or less; about 50 psig (345 kPa) or less; or about 30 psig (205kPa) or less and the temperature of the feedstock gas 112 can be about200° F. (93° C.) or less; about 150° F. (66° C.) or less; about 100° F.(38° C.) or about 30° F. (0° C.) or less.

The higher pressure oxygen containing gas may be introduced, mixed, orotherwise combined with the feedstock gas 112 either within the one ormore OCM reactors 102 or prior to the entry of either, or both, thehigher pressure oxygen containing gas and/or the feedstock gas 112 tothe one or more OCM reactors 102. The feedstock gas/oxygen containinggas mixture 110 can be treated to remove one or more contaminants priorto introduction to the one or more OCM reactors 102. Contaminantspresent in the feedstock gas 112 may be detrimental to the OCM catalystand/or the one or more OCM reactors 102 themselves and therefore theconcentration of these contaminants is reduced prior to introducing thefeedstock gas/oxygen containing gas mixture 110 to the one or more OCMreactors 102. For example, elemental sulfur or hydrogen sulfide may bepresent in concentrations ranging from trace amounts to double-digit mol% quantities within feedstock gas sources, such as extracted naturalgas. The presence of sulfur or hydrogen sulfide can promote theformation of corrosive sulfurous acid within the one or more OCMreactors 102 and therefore are most desirably removed from the feedstockgas 112, oxygen containing gas 106 or the feedstock gas/oxygencontaining gas mixture 110 prior to introducing the mixture to the oneor more OCM reactors 102. After removal of sulfur or hydrogen sulfide,the hydrogen sulfide content of the feedstock gas/oxygen containing gasmixture 110 can be about 20 ppm or less; about 10 ppm or less; about 5ppm or less; or more preferably about 1 ppm or less.

Additionally, the temperature of the feedstock gas/oxygen containing gasmixture 110 may be adjusted prior to introducing the mixture to the oneor more OCM reactors 102. The temperature can be adjusted to a desiredlevel to optimize the generation of preferred products such as ethylenewithin the one or more OCM reactors 102. In some instances, thetemperature of the feedstock gas/oxygen containing gas mixture 110 maybe adjusted in conjunction with one or more pretreatment steps, forexample desulfurization of the feedstock gas/oxygen containing gasmixture 110. Prior to entering the one or more OCM reactors 102, thetemperature of the feedstock gas/oxygen containing gas mixture 110 canbe about 1300° F. (700° C.) or less; about 1110° F. (600° C.) or less;about 930° F. (500° C.) or less; about 750° F. (400° C.) or less; about570° F. (300° C.) or less; or about 400° F. (200° C.) or less.

The OCM reactor 102 can include any vessel, device, system or structurecapable of converting at least a portion of the feedstock gas/oxygencontaining gas mixture 110 into one or more C₂ compounds using anoxidative coupling of methane process. The one or more OCM reactors 102can be one or more similar or dissimilar reactors or reactor typesarranged in series or parallel processing trains. The OCM process may becarried out in different types of commercially available reactorsincluding a fixed bed reactor where the combined methane/oxygen gasmixture is passed through a structured bed; a fluidized bed reactorwhere the combined methane/oxygen mixture is used to fluidize a solidcatalyst bed; and a membrane type reactor where the combinedmethane/oxygen mixture passes through an inorganic catalytic membrane.

The OCM reaction (2CH₄+O₂→C₂H₄+2H₂O) is exothermic (ΔH=−67 kcals/mole)and generally requires very high temperatures (>700° C.). As aconsequence, the OCM reactors 102 can be sized, configured, and/orselected based upon the need to dissipate the heat generated by the OCMreaction, for example in some embodiments, multiple, tubular, fixed bedreactors can be arranged in parallel to facilitate heat removal. In atleast some embodiments, at least a portion of the heat generated withinthe one or more OCM reactors 102 can be recovered, for example the heatcan be used to generate high pressure steam. Where co-located withprocesses requiring a heat input, at least a portion of the heatgenerated within the one or more OCM reactors 102 may be transferred,for example using a heat transfer fluid, to the co-located processes.Where no additional use exists for the heat generated within the one ormore OCM reactors 102, the heat can be released to the atmosphere, forexample using a cooling tower or similar evaporative cooling device.

In other embodiments, the one or more OCM reactors 102 may includemultiple adiabatic, fixed-bed, OCM reactors arranged in a cascadedseries, where the OCM product gas generated by a first OCM reactor isremoved and introduced to a second OCM reactor, subsequent cascaded OCMreactors can be similarly arranged. In at least some embodiments, theOCM product gas removed from each reactor may be cooled, for example ina horizontal or vertical tube boiler using boiler feed water to generatehigh pressure steam, prior to introduction to a subsequent OCM reactor.A multi-stage, cascaded OCM reactor arrangement advantageously providesthe ability to control the thermal profile through each OCM reactor andthrough the entire cascaded OCM reactor series. The ability to provideindependent reactor thermal profiling as well as thermal profilingthroughout all of the cascaded reactors can improve catalyst performanceand catalyst life as well as providing a degree of product selectivityin the OCM product gas 114.

In addition to a parallel configuration, multiple OCM reactors 102 maybe arranged in a serial configuration or even a combination of seriesand parallel configurations. In a multiple reactor configuration, theOCM reactors 102 can be similar or different in size, type, or designbased at least in part on process conversion and heat transferspecifications.

Chemical conversion is a measure of the quantity of reactants convertedvia a chemical reaction. Chemical selectivity is a measure of thequantity of a reactant converted to the desired product. For example,within the one or more OCM reactors, in addition to the desiredethylene, methane in the feedstock gas 112 will also be converted toundesirable or unwanted byproducts including, but not limited to, watervapor, oxides of carbon, and hydrogen. The ethylene selectivity of theone or more OCM reactors 102 is therefore a quantitative measure oftheir ability to convert methane in the feedstock gas 112 to ethylene inthe OCM product gas 114. Conversion and selectivity are dependent upon amultitude of factors, including but not limited to: reactor design,catalyst, and operating conditions.

Although other OCM catalysts can be disposed in at least a portion ofthe one or more OCM reactors 102, in at least some embodiments, at leasta portion of the OCM catalyst in at least a portion of the one or moreOCM reactors can include one or more nanowire-based OCM catalysts suchas those developed by Siluria Technologies Inc. (Palo Alto, Calif.) anddescribed in: U.S. patent application Ser. No. 13/115,082, filed May 24,2011, entitled “Nanowire Catalysts;” U.S. Provisional Patent ApplicationSer. No. 61/564,832, filed Nov. 29, 2011, entitled “Catalysts forPetrochemical Catalysis;” U.S. Provisional Patent Application Ser. No.61/564,834, filed Nov. 29, 2011, entitled “Nanowire Catalysts;” and U.S.Provisional Patent Application Ser. No. 61/564,836, filed Nov. 29, 2011,entitled “Polymer Templated Nanowire Catalysts”, all of which areincorporated in their entirety by reference as if reproduced in theirentirety herein. Using one or more nanowire based OCM catalysts withinthe one or more OCM reactors 102, the selectivity of the catalyst inconverting methane to desirable C₂ products can be about 10% or greater;about 20% or greater; about 30% or greater; about 40% or greater; about50% or greater; about 60% or greater; about 65% or greater; about 70% orgreater; about 75% or greater; about 80% or greater; or about 90% orgreater.

The one or more OCM reactors 102 provide an OCM product gas 114.Although variable based upon a multitude of process equipment, reactantand process conditions, the OCM product gas 114 can contain in additionto the desired and valued ethylene product: water vapor, methane,ethane, nitrogen, hydrogen, carbon oxides, small quantities of heavierhydrocarbons (hydrocarbons containing three or more carbon atoms),acetylene or inert compounds. In exemplary embodiments, the ethylenecontent of the OCM product gas 114 can be about 0.5 mol % or greater;about 1 mol % or greater; about 2 mol % or greater; about 5 mol % orgreater; about or more preferably about 7 mol % or greater and theethane content of the OCM product gas 114 can be about 0.5 mol % orgreater; about 1 mol % or greater; about 2 mol % or greater; about 5 mol% or greater; or more preferably about 7 mol % or greater. In at leastsome implementations, one or more inert gases, such as nitrogen, argon,or helium may be present in trace or larger quantities in the OCMproduct gas 114.

Considerable quantities of nitrogen can be present in the OCM productgas 114, particularly where air is used to supply all or a portion ofthe oxygen containing gas 106. Nitrogen is carried through the one ormore OCM reactors 102 as an inert compound and may therefore appear at arelatively high concentration in the OCM product gas 114. In contrast,the nitrogen content of the OCM product gas 114 can be relatively lowwhen purified oxygen, for example oxygen supplied by an air separationunit, is used to provide nearly all or all of the oxygen containing gas106. In exemplary embodiments, the nitrogen content of the OCM productgas 114 can be about 1 mol % or less; about 5 mol % or less; about 10mol % or less; about 25 mol % or less; about 40 mol % or less; or about60 mol % or less. As discussed more fully below, the nitrogen or similarchemically inert gases in the OCM product gas 114 can be usedadvantageously in the separation unit 104 to provide some or all of thecooling required to recover C₂ compounds from the OCM product gas 114.

Hydrogen is liberated from the unsaturated hydrocarbons formed in theOCM reaction and may be present as a potential byproduct in the OCMproduct gas 114. The hydrogen content of the OCM product gas 114 can beabout 4 mol % or less; about 3 mol % or less; about 2 mol % or less; ormore preferably about 1 mol % or less.

Carbon oxides, including carbon monoxide and carbon dioxide, form as aresult of the complete combustion of a portion of the hydrocarbonswithin the feedstock gas 112. Such combustion is an unintentionalconsequence of operating a high temperature hydrocarbon conversionprocess. In exemplary embodiments, the carbon oxide content of the OCMproduct gas 114 can be about 10 mol % or less; about 7 mol % or less; ormore preferably about 5 mol % or less.

Within the one or more OCM reactors 102, the conversion of methane toheavier hydrocarbons is less than 100%. As a consequence, unreactedmethane will be present in the OCM product gas 114. The quantity ofmethane within the OCM product gas will vary dependent upon the degreeof conversion achieved within the one or more OCM reactors 102. Inexemplary embodiments, the methane content of the OCM product gas 114can be about 95 mole percent (mol %) or less; about 90 mol % or less;about 80 mol % or less; about 70 mol % or less; about 60 mol % or less;about 40 mol % or less; about 30 mol % or less; about 20 mol % or less;or about 10 mol % or less.

Where air is used to provide at least a portion of the oxygen containinggas 106, argon will accumulate due to the recycle of at least a portionof the non-condensable gas (principally nitrogen) from the separationunit 104 to the OCM reactors 102. In at least some situations, the argoncontent of the OCM product gas 110 can be 0 mol % for pure oxygen feed,about 1 mol % or less; about 5 mol % or less.

The temperature and pressure of the OCM product gas 114 is dependentupon maintaining a temperature and pressure profile within the one ormore OCM reactors 102 that favors ethylene production while disfavoringthe production of less desirable or undesirable by-products. Uponexiting the one or more OCM reactors 102, the OCM product gas 114 can beat a pressure of about 200 psig (1380 kPa) or less; about 150 psig (1035kPa) or less; about 100 psig (690 kPa) or less; or more preferably about50 psig (345 kPa) or less. Due to the exothermic nature of the OCMreaction, post-cooling, for example by passing the OCM product gas 114across a boiler feedwater preheater proximate the one or more OCMreactors 102. In exemplary embodiments, upon exiting the one or more OCMreactors 102, the OCM product gas 114 can be at a temperature of about1750° F. (950° C.) or less; about 1650° F. (900° C.) or less; about1560° F. (850° C.) or less; about 1470° F. (800° C.) or less; about1380° F. (750° C.) or less; about 1300° F. (700° C.) or less; about1100° F. (590° C.) or less; about 900° F. (480° C.) or less; about 700°F. (370° C.) or less; or about 500° F. (260° C.) or less.

Upon exiting the one or more OCM reactors 102, the OCM product gas 114is at a relatively high temperature and a relatively low pressure.Recalling that the presence of one or more inert gasses (e.g., nitrogen)within the OCM product gas 114 can be used advantageously to reduce thetemperature within the separation unit 104, the pressure of the OCMproduct gas 114 may also be increased using the one or more compressors116 to provide a compressed OCM product gas 118. The temperature of theOCM product gas 114 may be adjusted using one or more pre-coolers (notshown) prior to introducing the OCM product gas 114 to the one or morecompressors 116. The temperature of the compressed OCM product gas 118may be reduced using one or more inter- or after-coolers afterintroducing the OCM product gas 114 to the one or more compressors 116.After exiting the one or more compressors 116, in exemplary embodimentsthe temperature of the compressed OCM product gas 118 can be about 150°F. (65° C.) or less; about 125° F. (52° C.) or less; about 100° F. (38°C.) or less; or about 75° F. (24° C.) or less. After exiting the one ormore compressors 116, in exemplary embodiments the pressure of thecompressed OCM product gas 118 can be at least about 100 psig (690 kPa);at least about 150 psig (1035 kPa); at least about 200 psig (1380 kPa);at least about 250 psig (1725 kPa); or at least about 300 psig (2070kPa).

The compressed OCM product gas 118 may be introduced to the separationunit 104. Within the separation unit 104, the mixed gasses within thecompressed OCM product gas 118 are separated to provide the methane richeffluent 120, the nitrogen rich effluent 122, and the C₂-rich effluent124. In at least some embodiments, the separation unit can use in wholeor in part, a cryogenic separation process to provide the methane richeffluent 120, the nitrogen rich effluent 122, and the C₂-rich effluent124.

Acetylene may be present in the OCM product gas at concentrations of upto about 0.1 mole percent (mol %); about 0.2 mol %; about 0.3 mol %;about 0.4 mol %; about 0.5 mol %; or about 0.75 mol %. All or a portionof any acetylene present in the OCM product gas 114 may be removed fromthe OCM product gas 114. In at least some instances, at least a portionof any acetylene present in the OCM product gas 114 may be converted toat least one more preferable chemical species. For example, at least aportion of any acetylene present in the OCM product gas 114 may beconverted to ethylene by passing all or a portion of the OCM product gas114 through an acetylene reactor where the acetylene is catalytically,selectively, hydrogenated. In another implementation, at least a portionof any acetylene present may be fully hydrogenated to provide ethane. Inyet another implementation, at least a portion of any acetylene presentmay be removed from the OCM product gas 114 and destroyed, for examplevia thermal combustion or oxidation in a controlled environment or viaflare.

The gases in at least a portion of the compressed OCM product gas 118may be adiabatically expanded to provide at least a portion of thecooling used in the cryogenic processes within the separation unit 104.In some embodiments, at least a portion of the gasses within theseparation unit 104 may be recompressed and re-adiabatically expanded toprovide additional cooling and, potentially, obviate the need forexternal refrigeration within the separation unit 104. Additionally, insome instances, at least a portion of the gasses within the separationunit 104 may be isentropically expanded, for example using aturboexpander, to provide mechanical work (e.g., a shaft output) usefulwithin the separation unit 104.

Within the separation unit 104, heavier hydrocarbon compounds, includingethane and ethylene are separated from the OCM product gas 118 toprovide the C₂-rich effluent 124 and a mixed nitrogen/methane containinggas. In some implementations, any acetylene present in the OCM productgas 118 may be at least partially removed using one or more systems,devices, or processes included in the separation unit 104. In otherimplementations, any acetylene present in the OCM Product gas 118 may beat least partially converted to one or more preferable chemical speciesusing one or more systems, devices, or processes included in theseparation unit 104. For example, all or a portion of the OCM productgas 118 or the C₂-rich effluent 124 may be passed through an acetylenereactor where at least a portion of the acetylene present in the OCMproduct gas 118 or the C₂-rich effluent 124 is selectively,catalytically hydrogenated to ethylene. Such acetylene removal orconversion may occur at any point in the separation unit 104, includingpreparatory to performing any separations (e.g., removal from the OCMproduct gas 118), after completion of the separations (e.g., removalfrom the C₂-rich effluent 124), at an intermediate stage of theseparations process, or any combination thereof.

The mixed nitrogen/methane containing gas can be separated in separationunit to provide the nitrogen-rich effluent 122 and the methane-richeffluent 120. At least a portion of the methane-rich effluent 120 can berecycled back to the feedstock gas 112 or to the one or more OCMreactors 102. The methane-rich effluent 120 consists primarily ofmethane with other compounds present in small quantities. In exemplaryembodiments, the methane content of the methane-rich effluent 120 can beabout 60 mole percent (mol %) or greater; 80 mol % or greater; about 85mol % or greater; about 90 mol % or greater; or more preferably about 95mol % or greater. As explained above, the quantity and the concentrationof nitrogen in the nitrogen-rich effluent 122 can depend upon the sourceused to supply the oxygen containing gas 106. In exemplary embodiments,the nitrogen content of the nitrogen-rich effluent 122 can be about 50mol % or greater; about 75 mol % or greater; about 85 mol % or greater;or more preferably about 90 mol % or greater. The C₂-rich effluent 124contains ethane, ethylene, and higher molecular weight hydrocarbons. Inexemplary embodiments, the ethane content of the C₂-rich effluent 124can be about 10 mol % or greater; about 20 mol % or greater; about 30mol % or greater; about 40 mol % or greater; about 50 mol % or greater;or more preferably about 60% or greater and the ethylene content of theC₂-rich effluent 124 can be about 10 mol % or greater; about 20 mol % orgreater; about 30 mol % or greater; about 40 mol % or greater; about 50mol % or greater; or more preferably about 60 mol % or greater.

FIG. 2 is a block flow diagram depicting an illustrative C₂ productionand separation process 200 having one or more oxidative coupling ofmethane (OCM) reactors 102, one or more separation units 104, and one ormore pretreatment units 202. The one or more pretreatment units 202 areuseful in removing contaminants and other undesirable compounds from thecompressed OCM product gas 118 to provide a cleaned, compressed, OCMproduct gas 204. For example, water and carbon dioxide, both of whichcan freeze during a cryogenic process within the one or more separationunits 104 may be removed from the compressed OCM product gas 118 in oneor more pretreatment units 202. Although depicted as a single entity inFIG. 2, the one or more pretreatment units 202 can include multiple unitoperations each targeting the reduction of the amount of one or morecontaminants present within the compressed OCM product gas 118.

The compressed OCM product gas 118 can be introduced to the one or morepretreatment units 202 to remove one or more undesired componentspresent in the compressed OCM product gas 118. For example, all or aportion of the carbon dioxide present within the compressed OCM productgas 118 can be removed within the one or more pretreatment units 202.Numerous methods of reducing the amount of carbon dioxide within thecompressed OCM product gas 118 may be used. For example, the carbondioxide level within the compressed OCM product gas 118 may be reducedby contacting the compressed OCM product gas 118 with a solutioncontaining one or more amines such as monoethanolamine (MEA). In atleast some embodiments at least a portion of the steam produced as abyproduct from the one or more OCM reactors can be used to facilitatethe removal of carbon dioxide 206 from the compressed OCM gas 118 toprovide the cleaned, compressed OCM product gas 204. For example,byproduct steam may be useful in the thermal regeneration of causticthat has been converted to calcium carbonate in a carbon dioxidescrubber. In exemplary embodiments, the carbon dioxide content of thecleaned, compressed OCM product gas 204 can be about 20 ppm or less;about 10 ppm or less; or more preferably about 5 ppm or less.

Additionally, all or a portion of the water 208 present in thecompressed OCM gas 118 as a water vapor can be removed within the one ormore pretreatment units 202. Numerous methods of reducing water vaporlevels within the compressed OCM product gas 118 may be used. Forexample, the amount of water in the form of water vapor within thecompressed OCM product gas 118 may be reduced using a thermal swingadsorption (TSA) process such as a multi-column TSA process enablingcontinuous water vapor removal and adsorbent bed regeneration. In atleast some embodiments at least a portion of the steam produced as abyproduct from the one or more OCM reactors can be used to facilitatethe regeneration of the adsorbent beds within a TSA process. Inexemplary embodiments, the water vapor content of the cleaned,compressed OCM product gas 204 can be about 0.05 mol % or less; about0.01 mol % or less; or more preferably about 0.001 mol % or less.

In at least some implementations, the pretreatment unit 202 may includeone or more systems, devices, or processes to optionally remove at leasta portion of any acetylene present in the OCM product gas 118. In otherimplementations, the pretreatment unit 202 may include one or moresystems, devices, or processes to optionally convert at least a portionof any acetylene present in the OCM product gas 118 to one or morepreferred chemical species. For example, in at least someimplementations, all or a portion of the OCM product gas 118 may bepassed through an acetylene reactor where at least a portion of theacetylene present in the OCM product gas 118 can be selectively,catalytically hydrogenated to ethylene.

FIG. 3 is a basic process flow diagram depicting a methane based C₂production and separation process 300 including a first separator 302providing the C₂-rich effluent 124 and a methane/nitrogen gas and asecond separator 304 providing the methane-rich effluent 124 and thenitrogen-rich effluent 122. In the embodiment illustrated in FIG. 3, thetemperature of the compressed OCM product gas 118 is reduced using oneor more heat exchangers 306. The temperature of the compressed OCMproduct gas 118 may be lowered through the use of an external coolingmedia, a relatively cool process stream, or combinations thereof.Reducing the temperature of the OCM product gas 118 will condense atleast a portion of the higher boiling point components in the compressedOCM product gas 118, including at least a portion of the C₂ and heavierhydrocarbons present in the compressed OCM product gas 118.

At least a portion of the condensed high boiling point components can beseparated from the compressed OCM product gas 118 using one or moreliquid/gas separators, such as knockout drums 308 to provide an OCMproduct gas condensate 310 and a compressed OCM product gas 312. The OCMproduct gas condensate 310 is introduced to the first separator 302 andat least a portion 314 of the compressed OCM product gas 312 can beintroduced to one or more turboexpanders 316. The isentropic expansionof the compressed OCM product gas 314 within turboexpanders 316 canproduce shaft work useful for driving one or more compressors or otherdevices in the separation unit 104. The isentropic expansion of thecompressed OCM product gas 314 with the turboexpanders reduces thetemperature of the compressed OCM product gas 318 that exits from theone or more turboexpanders. The compressed OCM product gas 318 from theone or more turboexpanders 316 is introduced to the first separator 302.

The first separator 302 can be any system, device or combination ofsystems and devices suitable for promoting the separation of C₂ andheavier hydrocarbons from a gas stream comprising mainly nitrogen andmethane. For example, cryogenic distillation at a relatively hightemperature may be used to promote the separation of C₂ and heavierhydrocarbons from a gas comprising mainly nitrogen and methane. TheC₂-rich effluent 124 is withdrawn from the first separator 302 and amixed nitrogen/methane containing gas mixture 320 is also withdrawn fromthe first separator 302. The nitrogen content of the nitrogen/methanecontaining gas mixture 320 withdrawn from the first separator 302 can beabout 95 mol % or less; about 85 mol % or less; about 75 mol % or less;about 55 mol % or less; about 30 mol % or less. The balance of thenitrogen/methane gas mixture 320 comprises principally methane withsmall quantities of hydrogen, carbon monoxide, and inert gases such asargon.

In at least some embodiments, the first separator 302 may be referred toas a “demethanizer” based on its ability to separate methane from C₂ andhigher hydrocarbons. An exemplary first separator 302 is provided by avertical distillation column operating at below ambient temperature andabove ambient pressure. The operating temperature and pressure withinthe first separator 302 can be established to improve the recovery ofthe desired C₂ hydrocarbons in the C₂-rich effluent 124. In exemplaryembodiments, the first separator 302 can have an overhead operatingtemperature of from about −260° F. (−162° C.) to about −180° F. (−118°C.); about −250° F. (−157° C.) to about −190° F. (−123° C.); about −240°F. (−151° C.) to about −200° F. (−129° C.); or more preferably fromabout −235° F. (−148° C.) to about −210° F. (−134° C.) and an bottomoperating temperature of from about −150° F. (−101° C.) to about −50° F.(−46° C.); about −135° F. (−93° C.) to about −60° F. (−51° C.); fromabout −115° F. (−82° C.) to about −70° F. (−57° C.); or more preferablyabout −100° F. (−73° C.) to about −80° F. (−62° C.). In exemplaryembodiments, the first separator 302 can be at an operating pressure offrom about 30 psig (205 kPa) to about 130 psig (900 kPa); about 40 psig(275 kPa) to about 115 psig (790 kPa); about 50 psig (345 kPa) to about95 psig (655 kPa); or more preferably about 60 psig (415 kPa) to about80 psig (550 kPa).

The temperature of at least a portion of the C₂-rich effluent 124 fromfirst separator 302 can be increased in one or more heat exchangers 322using a heat transfer fluid, a warm process flow stream, or acombination thereof. The one or more heat exchangers 322 can include anytype of heat exchange device or system including, but not limited to oneor more plate and frame, shell and tube, or the like. After exiting theone or more heat exchangers 322, in exemplary embodiments, thetemperature of the C₂-rich effluent 124 can be about 50° F. (10° C.) orless; about 25° F. (−4° C.) or less; about 0° F. (−18° C.) or less;about −25° F. (−32° C.) or less; or about −50° F. (−46° F.) or less andthe pressure can be about 130 psig (900 kPa) or less; about 115 psig(790 kPa) or less; about 100 psig (690 kPa) or less; or more preferablyabout 80 psig (550 kPa) or less.

The temperature of the nitrogen/methane containing gas mixture 320withdrawn from the first separator 302 can be lowered in one or moreheat exchangers 324 using one or more refrigerants, one or morerelatively cool process flows, or combinations thereof. The one or moreheat exchangers 324 can include any type of heat exchange device orsystem including, but not limited to one or more plate and frame, shelland tube, or the like. The cooled nitrogen/methane gas mixture 320exiting one or more heat exchangers 324 is introduced to the secondseparator 304.

In some embodiments a portion 326 of the OCM product gas 312 removedfrom the knockout drum 308 and not introduced to the one or moreturboexpanders 316 can be cooled using one or more heat exchangers 328.The one or more heat exchangers 328 can include any type of heatexchange device or system including, but not limited to one or moreplate and frame, shell and tube, or the like. The temperature of theportion 326 of the OCM product gas 312 can be decreased using one ormore refrigerants, one or more relatively cool process flows, orcombinations thereof. The cooled portion 326 of the OCM product gas 312,containing a mixture of nitrogen and methane is introduced to the secondseparator 304.

The second separator 304 can be any system, device or combination ofsystems and devices suitable for promoting the separation of methanefrom nitrogen. For example, cryogenic distillation at a relatively lowtemperature can be used to promote the separation of methane fromnitrogen in a gas stream. Conditions within the second separator 304promote the condensation of methane and the separation of liquid methanefrom the gaseous nitrogen within the second separator 304. The liquidmethane containing methane-rich effluent 120 is withdrawn as a liquidfrom the second separator 304 and the nitrogen-rich effluent 122 iswithdrawn as a gas from the second separator 304. An exemplary secondseparator 304 is provided by a vertical distillation column operatingsignificantly below ambient temperature and above ambient pressure. Theoperating temperature and pressure within the second separator 302 canbe established to improve the separation of liquid methane as themethane-rich effluent 120 from the gaseous nitrogen as the nitrogen-richeffluent 122. For example, the second separator 304 can have an overheadoperating temperature of from about −340° F. (−210° C.) to about −240°F. (−151° C.); about −330° F. (−201° C.) to about −250° F. (−157° C.);about −320° F. (−196° C.) to about −260° F. (162° C.); about −310° F.(−190° C.) to about −270° F. (−168° C.); or more preferably about −300°F. (−184° C.) to about −280° F. (−173° C.) and a bottom operatingtemperature of from about −280° F. (−173° C.) to about −170° F. (−112°C.); about −270° F. (−168° C.) to about −180° F. (−118° C.); about −260°F. (−162° C.) to about −190° F. (−123° C.); about −250° F. (−159° C.) toabout −200° F. (−129° C.); or more preferably about −240° F. (−151° C.)to about −210° F. (−134° C.). In exemplary embodiments, the secondseparator 304 can be at an operating pressure of from about 85 psig (585kPa) or less; about 70 psig (480 kPa) or less; about 55 psig (380 kPa)or less; or more preferably about 40 psig (275 kPa) or less.

The temperature of at least a portion of the methane-rich effluent 120from the second separator 304 can be increased using one or more heatexchangers 330. In at least some instances, one or more compressors maybe used to increase the pressure and temperature of the methane-richeffluent 120 from the second separator 304 prior to recycling at least aportion of the compressed methane-rich effluent 304 to the feedstockgas/oxygen containing gas mixture 110. The one or more heat exchangers330 can include any type of heat exchange device or system including,but not limited to one or more plate and frame, shell and tube, or thelike. The temperature of the methane-rich effluent 120 may be increasedin heat exchangers 330 using a heat transfer fluid, a warm process flow,or a combination thereof. After exiting the one or more heat exchangers330, in exemplary embodiments the temperature of the methane-richeffluent 120 can be about 125° F. (52° C.) or less; about 100° F. (38°C.) or less; or more preferably about 90° F. (32° C.) or less and thepressure of the methane-rich effluent 120 can be about 150 psig (1035kPa) or less; about 100 psig (690 kPa) or less; or more preferably about50 psig (345 kPa) or less. In an embodiment in accordance with FIG. 3,at least a portion of the methane-rich effluent 120 can be recycled tothe feedstock gas 112, the feedstock gas/oxygen containing mixture 110,the compressed oxygen containing gas, or to the one or more OCM reactors102.

The temperature of at least a portion of the nitrogen-rich effluent 122can be increased using one or more heat exchangers 332. The one or moreheat exchangers 332 can include any type of heat exchange device orsystem including, but not limited to one or more plate and frame, shelland tube, or the like. The temperature of the nitrogen-rich effluent 122may be increased in heat exchangers 332 using a heat transfer fluid, awarm process flow, or a combination thereof. After exiting the one ormore heat exchangers 332, in exemplary embodiments, the temperature ofthe nitrogen-rich effluent 122 can be about 125° F. (52° C.) or less;about 100° F. (38° C.) or less; or more preferably about 90° F. (32° C.)or less and the pressure of the nitrogen-rich effluent 122 can be about150 psig (1035 kPa) or less; about 100 psig (690 kPa) or less; or morepreferably about 50 psig (345 kPa) or less.

Although described above for brevity and clarity as independent heatexchange devices, the one or more heat exchangers 306, 322, 324, 328,330, and 332 may be integrated into one or more composite heat exchangedevices permitting, where appropriate, heat exchange between processflows of differing temperatures.

FIG. 4 is a basic process flow diagram depicting a methane based C₂production and separation process 400 including compressed OCM productgas 114 pretreatment, a first separator 302 providing the C₂-richeffluent 124 and a methane/nitrogen gas and a second separator 304providing the methane-rich effluent 124 and the nitrogen-rich effluent122. In some embodiments, the compressed OCM product gas 114 can beintroduced to a carbon dioxide removal treatment system 402. Carbondioxide removal treatment system can comprise systems suitable forremoving carbon dioxide from OCM product gas 114. In some embodiments,an ethanolamine-based carbon dioxide removal process can be used toscrub carbon dioxide from the compressed OCM product gas 114. The spentethanolamine solution can be regenerated via heating thereby providing arecyclable carbon dioxide scrubbing solution. In other embodiments, acaustic-based carbon dioxide removal process can be used to scrub carbondioxide from the compressed OCM product gas 114. In some instances, thesodium carbonate formed by scrubbing the carbon dioxide from thecompressed OCM product gas 114 can be reacted with calcium hydroxide toform calcium carbonate and regenerate the caustic for recycle to thecarbon dioxide removal treatment system 402.

The compressed OCM product gas 114 can also be introduced to a waterremoval system 404 that includes systems for removing water from OCMproduct gas 114. In some embodiments, the water removal system caninclude a thermal swing adsorption (TSA) system having at least two TSAcolumns to provide continuous water removal capability. Further detailsof exemplary TSA water removal systems have been described above.

FIG. 5 is a process flow diagram depicting a separation process 500useful within one or more separation units 104 to separate the cleaned,compressed OCM product gas 204, according to one illustrated embodiment.As depicted in FIG. 5, heat exchange between various process streams isused to provide process heating and cooling as needed. Importantly, theadiabatic expansion of one or more process gasses coupled with the useof process heat exchange can minimize or even eliminate the requirementfor the supply of external refrigeration to the separation process 500.In some instances, for example where purified or separated oxygenprovides at least a portion of the oxygen containing gas 106,insufficient gas volume within the separation unit 104 may serve tolimit the cooling effect realized by the adiabatic expansion of processgas within the separation unit 104. In such embodiments, externalcooling, for example cooling from a cryogenic process providing theoxygen containing gas 106 may be used to provide at least a portion ofthe cooling within the separation unit 104.

In at least some embodiments, the pressure of the methane-rich effluent120 withdrawn from the second separator 304 may be adjusted to provide amethane-rich effluent at two or more pressures. Such an arrangement maybe advantageous for example, when a first portion 502 of themethane-rich effluent 120 is intended for distribution within acommercial or industrial distribution network operating at a relativelylow pressure and a second portion 504 of the methane-rich effluent 120is intended for injection into a transport pipeline operating at arelatively high pressure. For example, the pressure of the first portion502 of the methane-rich effluent 120 may be reduced to a pressure offrom about 5 psig (35 kPa) to about 30 psig (205 kPa) by passing portion502 of the methane-rich effluent 120 through a pressure reduction devicesuch as a pressure reducing valve 506. The pressure of the secondportion 504 of the methane-rich effluent 120 may be increased to apressure of from about 30 psig (205 kPa) to about 100 psig (690 kPa) bypassing portion 504 of the methane-rich effluent 120 through a pressureincreasing device such as a fluid mover 508.

The cleaned, compressed OCM product gas 204 is introduced to theseparation process 500. Recall, the OCM product gas exiting the one ormore OCM reactors 102 is at an elevated temperature. While the OCMproduct gas is cooled, the temperature of the OCM product gas enteringthe separation process 500 remains at a relatively warm temperature, forexample between about 50° F. (10° C.) and 150° F. (66° C.). Conversely,the C₂-rich effluent 124 withdrawn from the first separator 302 istypically at a relatively cool temperature, for example between about−150° F. (−101° C.) and about −80° F. (−62° C.). The first and secondportions 502, 504 of the methane-rich effluent 120 and the nitrogen-richeffluent 122 withdrawn from the second separator 304 are also typicallyat relatively cool temperatures, for example between −340° F. (−207° C.)and about −170° F. (−112° C.). By thermally contacting the relativelywarm cleaned, compressed OCM product gas 204 with the relatively coolC₂-rich effluent 124, first and second portions 502, 504 of themethane-rich effluent 120 and the nitrogen-rich effluent 122 in a firstheat exchange device 510, the temperature of the cleaned, compressed OCMproduct gas 204 can be decreased and the temperature of the effluentstreams increased. The first heat exchange device 510 can be any type,size, or shape heat exchange device capable of transferring heat betweenthree or more components.

Recall from FIG. 3 the OCM product gas 312 removed from the knockoutdrum 308 can be apportioned or otherwise separated into a first portion314 that is introduced to the one or more turboexpanders 316 and asecond portion 326 that is ultimately introduced to the second separator304. The adiabatic expansion of the cleaned, compressed OCM product gas204 within the knockout drum 308 reduces the temperature of the OCMproduct gas 312 exiting drum 308. Thus, the temperature of the secondportion of the OCM product gas 326 will be at a lower temperature thanthe cleaned, compressed OCM product gas 204 entering knockout drum 308.

By thermally contacting the relatively warm cleaned, compressed OCMproduct gas 204 and the second portion of the OCM product gas 326 withthe relatively cool first and second portions 502,504 of themethane-rich effluent 120 and the nitrogen-rich effluent 122 in a secondheat exchange device 512, the temperature of the cleaned, compressed OCMproduct gas 204 and the second portion of the OCM product gas 326 can befurther decreased and the temperature of the effluent streams increased.The second heat exchange device 512 can be any type, size, or shape heatexchange device capable of transferring heat between three or morecomponents.

Cooling the second portion of the OCM product gas 326 can form a secondOCM product gas condensate within the second portion of the OCM productgas 326. The second portion of the OCM product gas 326 can be introducedto a liquid/gas separation device, such as a knockout drum 514 where thesecond OCM product gas condensate 516 is removed and returned to thefirst separator 302, for example as a reflux to the first separator 302.The OCM product gas 518 is withdrawn from the drum 514 and introduced tothe second separator 304.

In some embodiments, by thermally contacting the relatively warm OCMproduct gas 518 and the nitrogen/methane gas mixture 320 withdrawn fromthe first separator 302 with the relatively cool first and secondportions 502, 504 of the methane-rich effluent 120 and the nitrogen-richeffluent 122 from second separator 304 in a third heat exchange device520, the temperature of the OCM product gas 518 and the nitrogen/methanegas mixture 320 can be further decreased and the temperature of theeffluent streams increased. The third heat exchange device 512 can beany type, size, or shape heat exchange device capable of transferringheat between three or more components.

In at least some embodiments, the pressure of the C₂-rich effluent 124withdrawn from the first separator 302 can be increased, for examplethrough the use of one or more fluid movers 522. The C₂-rich effluent124 contains a mixture of ethane, ethylene and heavier hydrocarbons suchas propane, butane, pentane and hexane. In at least some embodiments,all or a portion of the C₂-rich effluent 124 can be fractionated orotherwise separated, for example within a C₂ separation process orcolumn (e.g. a “de-ethanizer”) to provide at least an ethylene-richeffluent and an ethane-rich effluent. The ethylene-rich effluent canprovide either a feedstock to a subsequent process or a fungibleproduct. All or a portion of the ethane may be recycled back to thefeedstock gas 112. FIG. 6 is a process flow diagram depicting anotherseparation process 600 within one or more separation units 104 toseparate the cleaned, compressed OCM product gas 204 into desiredfractions, according to one illustrated embodiment. As depicted in FIG.6, in addition to heat exchange between various process streams toprovide process heating and cooling as needed, heat exchange is alsouseful for providing one or more reboiler loops (i.e., thermal energyinputs) to the first separator 302 and the second separator 304.Additionally, the nitrogen/methane gas mixture 320 from the firstseparator 302 and the OCM product gas 518 from knockout drum 514 mayalso provide one or more thermal contributions to the third heatexchanger 520.

In at least some embodiments, the OCM product gas 518 withdrawn from theknockout drum 514 is introduced to a second knockout drum 602.Non-condensed OCM product gas 604 is withdrawn from the second knockoutdrum 602. The temperature of the OCM product gas 604 can be reduced inthe third heat exchanger 520 prior to introducing the OCM product gas604 to the second separator 304. Any OCM product gas condensate 606 inthe second knockout drum 602 is withdrawn and introduced to a thirdknockout drum 608.

In at least some embodiments, the nitrogen/methane gas mixture 320 canbe withdrawn from the first separator 302 and introduced to the thirdheat exchanger 520 where a portion of the nitrogen/methane gas mixture320 can condense. Any condensate present in either or both thenitrogen/methane gas mixture 320 and the OCM product gas condensate 606are separated in the third knockout drum 608. The gas 610 within thethird knockout drum 608, comprising the nitrogen/methane gas mixture 320and any OCM product gas from the OCM product gas condensate 606 arewithdrawn from the third knockout drum 608. The temperature of therelatively warm gas 610 is increased using the third heat exchanger 520prior to introducing the gas 610 to the second separator 304. Similarly,relatively warm condensate 612 from the third knockout drum 608 iswithdrawn from the drum 608 and the temperature of the condensate 612increased using the third heat exchanger 520 prior to introducing thecondensate 612 to the second separator 304.

In some embodiments, the thermal efficiency of the separation unit 104may be improved by the transfer of thermal energy (i.e. heat) from thefirst heat exchanger 510 and the second heat exchanger 512 to the firstseparator 302 via reboiler loops 614 and 616, respectively. Similarly,additional thermal efficiency may be realized by the transfer of thermalenergy (i.e. heat) from the third heat exchanger 520 to the secondseparator 304 via reboiler loop 618. In some embodiments, the thermalenergy may be transferred between the heat exchangers and the separatorsusing a closed loop heat transfer fluid. In other embodiments, theliquid present in the separator may be withdrawn and passed through therespective heat exchanger. In yet other embodiments, a portion of thefirst and second heat exchangers 510, 512 may be partially or completelydisposed within the first separator 302 and a portion of the third heatexchanger 520 may be partially or completely disposed within the secondseparator 304.

The process flow diagram shown in FIG. 7 provides a more detailedbreakdown of the thermal conservation process 700 that occurs in thefirst heat exchanger 510, the second heat exchanger 512, and the thirdheat exchanger 520, according to one implementation. The thermaltransfer processes occurring in each of the first heat exchanger 510,the second heat exchanger 512, and the third heat exchanger 520 aredescribed in greater detail in Tables 1, 2, and 3 that follow. In thefollowing tables, the term “hot stream” refers to a gas, liquid, orcombination thereof whose thermal energy or temperature decreases as thegas, liquid, or combination thereof passes through the heat exchanger.In the following tables, the term “cold stream” refers to a gas, liquid,or combination thereof whose thermal energy or temperature increases asthe gas, liquid, or combination thereof passes through the heatexchanger. For convenience and ease of description, each of heatexchangers 510, 512, and 520 are broken into a number of thermal cellsin the following tables. One of ordinary skill in the chemicalengineering art would readily appreciate that such cells may be freelyadded, removed or changed between heat exchangers to provide alternatelevels of thermal conservation provided by the thermal conservationprocess 700. Although only one cold stream and one hot stream are shownas included in each thermal cell, one of ordinary skill in the chemicalengineering arts would readily appreciate that more than two streams(e.g., one hot stream and two cold streams, etc.) could be readilypassed through a single thermal cell.

TABLE 1 HEAT EXCHANGER 510 THERMAL CELLS Cell ID # Cold Gas/Liquid # HotGas/Liquid 710a 122 Nitrogen rich effluent 204 OCM product gas 710b 5021^(st) methane rich effluent 204 OCM product gas 710c 504 2^(nd) methanerich effluent 204 OCM product gas 710d 124 C₂-rich effluent 204 OCMproduct gas 710e 614 302 Reboiler loop 204 OCM product gas

TABLE 2 HEAT EXCHANGER 512 THERMAL CELLS Cell ID # Cold Gas/Liquid # HotGas/Liquid 712a 122 Nitrogen rich effluent 204 OCM product gas 712b 5021^(st) methane rich effluent 204 OCM product gas 712c 504 2^(nd) methanerich effluent 204 OCM product gas 712d 616 302 Reboiler loop 204 OCMproduct gas 712e 122 Nitrogen rich effluent 326 OCM product gas fraction712f 502 1^(st) methane rich effluent 326 OCM product gas fraction 712g504 2^(nd) methane rich effluent 326 OCM product gas fraction

TABLE 3 HEAT EXCHANGER 520 THERMAL CELLS Cell ID # Cold Gas/Liquid # HotGas/Liquid 720a 122 Nitrogen rich effluent 610 K/O drum gas 720b 122Nitrogen rich effluent 518 OCM product gas 720c 122 Nitrogen richeffluent 604 Non-cond. OCM prod. gas 720d 504 2^(nd) methane richeffluent 320 N₂/CH₄ gas mixture 720e 504 2^(nd) methane rich effluent612 Condensate 720f 502 1^(st) methane rich effluent 612 Condensate 720g122 Nitrogen rich effluent 612 Condensate

The block flow diagram 800 depicted in FIG. 8 shows an optionalpost-treatment system 802 to which at least a portion of the C₂-richeffluent 124 is introduced. In at least some instances, thepost-treatment system 802 may include any number of unit operations. Forexample, the post-treatment system 802 may include one or more devices,systems, or processes to provide an ethylene-rich effluent 804. Such anethylene-rich effluent 804 may be useful in any number of subsequentprocesses, for example an oligomerization or catalytic polymerizationprocess to produce a liquid gasoline product commonly referred to aspolygas.

In at least some implementations, the post-treatment system 802 includesany number of systems, devices, or processes for reducing the quantityof any acetylene in the C₂-rich effluent 124. In at least somesituations, such reduction may occur by removing as an acetylene-richeffluent 806 at least a portion of any acetylene present in the C₂-richeffluent 124. In at least some situations, such reduction may occur byconverting at least a portion of any acetylene present in the C₂-richeffluent 124 to one or more preferred chemical species. In at least someimplementations, the acetylene concentration in the C₂-rich effluent 124can be reduced to less than about 1 part per million by volume (ppmv);less than about 3 ppmv; less than about 5 ppmv; or less than about 10ppmv after removal or conversion in the post-treatment system 802.

In at least some instances, the C₂-rich effluent 124 may be furtherseparated the post-treatment system 802. The C₂-rich effluent 124 mayinclude a number of chemical species that includes a mixture of ethane,ethylene, and C₃₊ hydrocarbons. In at least some implementations, theethane, ethylene, and C₃₊ hydrocarbons may be partially or whollyseparated or otherwise isolated in the post-treatment system 802. Theseparation of the C₂-rich effluent 124 into a ethane, ethylene, and C₃₊can provide at least the ethylene-rich effluent 804, an ethane-richeffluent 808, and a C₃₊-rich effluent 810. In at least someimplementations, all or a portion of the C₂-rich effluent 124 may beintroduced to one or more systems, devices, or processes in whichethylene may be segregated, removed or otherwise isolated to provide theethylene-rich effluent 804. In at least some instances, theethylene-rich effluent 804 may include the overhead product of adistillation or cryogenic distillation process, for example adistillation process including at least one distillation column that iscolloquially known within the chemical arts as a “C₂ Splitter” thatoperates at a reduced temperature and an elevated pressure. In suchinstances, the ethylene-rich effluent 804 so produced may have anethylene concentration of about 75 mole percent (mol %) or more; about80 mol % or more; about 85 mol % or more; about 90 mol % or more; about95 mol % or more; about 99 mol % or more; or about 99.9 mol % or more.

In at least some implementations, the mixture of ethane and C₃₊hydrocarbons remaining after the removal of at least a portion of theethylene present in the C₂-rich effluent 124 may be introduced to one ormore systems, devices, or processes in which all or a portion of theethane may be segregated, removed or otherwise isolated to provide theethane-rich effluent 808. In at least some instances, the ethane-richeffluent 808 may include at least a portion of the overhead product of adistillation or cryogenic distillation process, for example adistillation process that includes at least one distillation columnoperating at a reduced temperature and an elevated pressure. In suchinstances, the ethane-rich effluent 808 so produced may have an ethaneconcentration of about 75 mole percent (mol %) or more; about 80 mol %or more; about 85 mol % or more; about 90 mol % or more; about 95 mol %or more; about 99 mol % or more; or about 99.9 mol % or more. In suchinstances, the C₃₊-rich effluent 810 may include at least a portion ofthe bottoms from the ethane separation process. In such instances, theC₃₊-rich effluent 810 so produced may have a C₃₊ hydrocarbonconcentration of about 75 mole percent (mol %) or more; about 80 mol %or more; about 85 mol % or more; about 90 mol % or more; about 95 mol %or more; about 99 mol % or more; or about 99.9 mol % or more.

Prophetic Example

Referring to FIG. 2, the following prophetic example illustrates acompositional analysis of an exemplary OCM process in accordance withembodiments disclosed herein with separation of the C₂-rich effluent124, the nitrogen-rich effluent 122 and the methane-rich effluent 120.

Flow C₁ N₂ O₂ H₂O CO CO₂ C₂ C₂═ Ref# (klb/hr) Mol % Mol % Mol % Mol %Mol % Mol % Mol % Mol % 106 36.4 0 78 21 0 0 0 0 0 112 5.8 99 0 0 0 0 00 0 110 56.9 48 39 10 0 0 0 0 0 114 56.9 37 39 0 12 1.1 2.4 2 1.8 20446.9 44 47 0 0 1.3 0 2.4 2.1 120 15.5 97 0.5 0 0 0.5 0 0 0 122 28.6 1 910 0 2.2 0 0 0 124 2.8 1 0 0 0 0 0 52 43

Although described in the context of an oxidative coupling of methane(OCM) process, the disclosed systems and methods can be applied to theseparation of a similarly composed C₂-rich effluent 124 from processessimilar to or different from the OCM production process described indetail herein. As an example, another process providing a gaseouseffluent similar to the OCM product gas 114 is provided by an oxidativedehydrogenation of ethane to form ethylene using air as the oxygencomprising feed gas.

The invention claimed is:
 1. A process for producing hydrocarboncompounds via oxidative coupling of methane (OCM), comprising: combininga feedstock gas comprising methane with an oxygen-containing gascomprising oxygen to produce a combined gas; bringing the combined gasin contact with an OCM catalyst in an OCM reactor to generate an OCMreactor effluent stream that includes ethylene and unreacted methane;separating the OCM reactor effluent stream into at least a first stream,a second stream, and a third stream; expanding at least a portion of thefirst stream substantially isentropically to produce an expanded firststream and expanding at least a portion of the third steam substantiallyisentropically to produce an expanded third stream; directing theexpanded first stream and the second stream to a separations unit toseparate at least a portion of a mixture of the expanded first streamand the second stream into a first effluent stream comprising ethyleneand a second effluent stream comprising unreacted methane, wherein theseparations unit is operated at a first temperature that is less than25° C. and at a first pressure that is greater than 1 atmosphere; andseparating at least a portion of the second effluent stream and at leasta portion of the expanded third stream in an additional separations unitto provide a methane-rich effluent and a nitrogen-rich effluent.
 2. Theprocess of claim 1, further comprising: compressing the at least aportion of the mixture of the expanded first stream and the secondstream prior to separating the at least a portion of the mixture of theexpanded first stream and the second stream into the first effluentstream and the second effluent stream.
 3. The process of claim 2,further comprising: increasing a pressure of the at least a portion ofthe mixture of the expanded first stream and the second stream to atleast 200 pounds per square inch gauge (psig) prior to separating the atleast a portion of the mixture of the expanded first stream and thesecond stream and the second effluent stream.
 4. The process of claim 1,further comprising: condensing at least a portion of the second streamto produce a condensed second stream.
 5. The process of claim 4, whereinthe condensing comprises flashing the at least a portion of the secondstream to a lower pressure.
 6. The process of claim 4, furthercomprising: prior to the condensing, performing at least any two of (i)reducing a water content of the at least a portion of the second streamto about 0.001 mole percent (mol %) or less, (ii) reducing a carbondioxide content of the at least a portion of the second stream to about5 parts per million by volume (ppmv) or less, or (iii) reducing anacetylene content of the at least a portion of the second stream toabout 1 part per million by volume (ppmv) or less.
 7. The process ofclaim 4, further comprising: separating at least a portion of theexpanded first stream and at least a portion of the condensed secondstream in the separations unit to provide at least a portion of thefirst effluent stream.
 8. The process of claim 1, wherein the additionalseparations unit is operated at a second temperature that is less than25° C. and at a second pressure that is greater than 1 atmosphere,wherein the second pressure is lower than the first pressure; andwherein the second temperature is lower than the first temperature. 9.The process of claim 1, further comprising: adding at least a portion ofthe methane-rich effluent to the feedstock gas prior to or during thecombining the feedstock gas with the oxygen-containing gas.
 10. Theprocess of claim 1, wherein the first effluent stream comprises at leastabout 90 mole percent (mol %) C₂ and higher hydrocarbon compounds.
 11. Aprocess for producing hydrocarbon compounds via oxidative coupling ofmethane (OCM), comprising: combining a feedstock gas comprising methanewith an oxygen-containing gas comprising oxygen to produce a combinedgas; bringing the combined gas in contact with an OCM catalyst in an OCMreactor to generate an OCM reactor effluent stream that includesethylene and unreacted methane; separating the OCM reactor effluentstream into at least a first stream, a second stream and a third stream;expanding at least a portion of the first stream in a firstturboexpander to produce an expanded first stream and expanding at leasta portion of the third stream in a second turboexpander to produce anexpanded third stream; directing the expanded first stream and thesecond stream to a separations unit to separate at least a portion of amixture of the expanded first stream and the second stream into a firsteffluent stream comprising ethylene and a second effluent streamcomprising the unreacted methane, wherein the separations unit isoperated at a first temperature that is less than 25° C. and at a firstpressure that is greater than 1 atmosphere; and separating at least aportion of the second effluent stream and at least a portion of theexpanded third stream in an additional separations unit to provide amethane-rich effluent and a nitrogen-rich effluent.
 12. The process ofclaim 11, further comprising: compressing the at least a portion of themixture of the expanded first stream and the second stream prior toseparating the at least a portion of the mixture of the expanded firststream and the second stream into the first effluent stream and thesecond effluent stream.
 13. The process of claim 12, further comprising:increasing a pressure of the at least a portion of the mixture of theexpanded first stream and the second stream to at least 200 pounds persquare inch gauge (psig) prior to separating the at least a portion ofthe mixture of the expanded first stream and the second stream into thefirst effluent stream and the effluent stream.
 14. The process of claim11, further comprising: condensing at least a portion of the secondstream to produce a condensed second stream.
 15. The process of claim14, wherein the condensing comprises flashing the at least a portion ofthe second stream to a lower pressure.
 16. The process of claim 14,further comprising: prior to the condensing, performing at least any twoof (i) reducing a water content of the at least a portion of the secondstream to about 0.001 mole percent (mol %) or less, (ii) reducing acarbon dioxide content of the at least a portion of the second stream toabout 5 parts per million by volume (ppmv) or less, or (iii) reducing anacetylene content of the at least a portion of the second stream toabout 1 part per million by volume (ppmv) or less.
 17. The process ofclaim 14, further comprising: separating at least a portion of theexpanded first stream and at least a portion of the condensed secondstream in the separations unit to provide at least a portion of thefirst effluent stream.
 18. The process of claim 11, wherein theadditional separations unit is operated at a second temperature that isless than 25° C. and at a second pressure that is greater than 1atmosphere, wherein the second pressure is lower than the firstpressure; and wherein the second temperature is lower than the firsttemperature.
 19. The process of claim 11, further comprising: adding atleast a portion of the methane-rich effluent to the feedstock gas priorto or during the combining the feedstock gas with the oxygen-containinggas.
 20. The process of claim 11, wherein the first effluent streamcomprises at least about 90 mole percent (mol %) C₂ and higherhydrocarbon compounds.